A Google search for “keynes” + “The market can remain irrational longer than you can remain solvent” turns up more than 150,000 hits, including this one and this one, from recent days.

Meanwhile, the new book “Safety Net” by James K. Glassman hangs its introduction on Keynes’ supposed riposte to one of his critics who accused him of being a flip-flopper: “When the facts change, I change my mind. What do you do, sir?” Hedge-fund managers Whitney Tilson and Glenn Tongue invoked the same quotation this week to explain why they are no longer shorting Netflix. (A Google search on this purported bit of Keynespeak turns up around 441,000 hits.)

There’s just one problem with both these quotations: No one can point to a primary source proving that Keynes ever uttered them.

In an e-mail in 2003, Keynes’ most authoritative biographer, Lord Robert Skidelsky, told investment advisor William Bernstein that he believed they were “both apocryphal.” This week I asked another renowned expert on Keynes, Donald Moggridge of the University of Toronto, if he could identify the source of either of the oft-quoted remarks. “The simple answer,” Prof. Moggridge replied by e-mail, “is there is no evidence.”

So here’s our challenge: MarketBeat will pay ten increasingly Keynesian U.S. dollars (yes, $10 in rapidly depreciating legal tender) to the first reader who can point to an irrefutable primary source proving that the godfather of fiscal intervention ever said either of these things. Find the sources for both and we’ll pay you $20.

If you hurry up, the $20 might even still be worth $20 by the time our envelope arrives in your mailbox. But we’re betting we never have to pay out a single sawbuck: We think both quotations are bogus.

Until then, would everybody please stop putting words in the old boy’s mouth?